(a) Field of the Invention
The present invention relates to a cathode ray tube, and more particularly, to an electron gun for a cathode ray tube having an scanning velocity modulation coil.
(b) Description of the Related Art
A cathode ray tube (CRT) typically includes a panel, a funnel, and a neck, which are integrally fused to define an exterior of the CRT. A phosphor screen is formed on an interior surface of the panel. Also, an electron gun is mounted within the neck, the electron gun emitting electron beams toward the phosphor screen. The funnel is positioned between the panel and the neck, and has a deflection yoke mounted to an outer circumference thereof for deflecting the electron beams emitted from the electron gun.
A configuration in which a scanning velocity modulation (SVM) coil is mounted on the neck of the CRT is well known. The SVM coil synchronizes a position of the electron beams passing through each electrode of the electron gun with image signals applied to the CRT to improve the resolution around edges of the image realized on the phosphor screen. The SVM coil is generally comprised of two saddle-shaped coils that are mounted in series and opposing each other on the neck of the CRT. Signals obtained by taking the second derivative of brightness signals of the image signals are applied to the SVM coil.
A CRT utilizing an SVM coil will now be described with reference to FIG. 7, which is an enlarged, partial sectional view of a neck of a conventional projection-type CRT.
An electron gun 10 that emits an electron beam is mounted in a neck 116. The electron gun 10 emits the electron beam in a rightward direction (in the drawing). The electron gun 10 includes a cathode 110 that emits the electron beam; a plurality of grid electrodes G1, G2, G3, G4, and G5 (hereinafter referred to as first, second, third, fourth, and fifth grid electrodes, respectively) that converge and accelerate the electron beams emitted from the cathode 110; and a bead glass 112 that fixes the first, second, third, fourth, and fifth grid electrodes G1, G2, G3, G4, and G5 in an aligned arrangement.
The first and second grid electrodes G1 and G2 have a short length in an axial direction Z of the CRT, while the third and fourth grid electrodes G3 and G4 are cylindrical and have a long length in the axial direction Z relative to the first and second grid electrodes G1 and G2. The fourth grid electrode G4 acts as a focusing electrode that converges the electron beams.
An SVM coil 114 is mounted to an outer circumference of the neck 116 at a position corresponding to the location of the third and fourth grid electrodes G3 and G4. That is, the SVM coil 114 is mounted to the neck 116 overlapping an area corresponding to the location between the third and fourth grid electrodes G3 and G4, and extending a predetermined distance in both directions.
In the CRT structured as in the above, the SVM coil 114 generates a magnetic field to control the landing of the electron beams on desired locations of a phosphor screen (not shown), that is, to control the scanning of the electron beams. However, the electron gun 10 reduces the ability of the electron beams to be scanned. In more detail, good results with respect to the magnetic field (generated by the SVM coil 114) controlling the electron beams are obtained only when all obstructions are removed as much as possible to allow the full effect of the magnetic field. However, because of the way in which the fourth grid electrode G4 is structured, the magnetic field is not able to directly act on the electron beams and is partially blocked reducing the strength of the magnetic field. Therefore, the landing of the electron beams cannot be precisely controlled. Further, when the magnetic field is partially blocked by the fourth grid electrode G4, an eddy current is generated on a surface of the fourth grid electrode G4 such that the magnetic field acting on the electron beams is further weakened. Here, the eddy current is proportional to a surface area of the electrode that blocks the magnetic field.
Although it is possible to improve a sensitivity of the magnetic field by varying a position of the SVM coil 114 on the neck 116, it is difficult to change the position of the SVM coil 114 since its mounting position is determined when designing the electron gun 10. Accordingly, in order to increase the sensitivity of the magnetic field, a number of windings of the SVM coil 114 or an amount of current flowing therethrough must be increased to strengthen the magnetic field. However, increasing the number of windings makes the SVM coil 114 larger and increasing the current increases the amount of power consumed by the SVM coil 114.
In an attempt to remedy these problems, Japanese Laid-open Patent No. Showa 55-146847 discloses a CRT in which an electron gun corresponding to a position of an SVM coil is realized through at least two separated electrodes with a predetermined gap between the electrodes. The SVM coil is mounted to an outer circumference of a neck corresponding to a position of the gap such that a magnetic field generated by the SVM coil passes through the gap.
Although sensitivity of the magnetic field can be enhanced in direct proportion to gap size, increases in gap size weaken the ability of the separated electrodes to converge electron beams as a result of an electrical field that enters from an exterior of the electrodes (e.g., an electrical field formed by a connector that electrically connects the separated electrodes). Therefore, it is not possible to make the gap large enough to realize sufficient improvements in magnetic field sensitivity of the SVM coil. That is, there are limits to how much the sensitivity of the magnetic field of the SVM coil can be increased.
Japanese Laid-open Patent No. Heisei 8-115684 discloses an electron gun in which electrodes of the electron gun corresponding to where an SVM coil is mounted are separated to form a gap, and a shield electrode is mounted to an end of the electrodes. By the operation of the shield electrode, a focus deterioration caused by the entrance of an external electrical field into the gap is prevented, and a reduction in the amount of an eddy current generated is realized such that a sensitivity of the magnetic field of the SVM coil is improved. However, this configuration has limitations as to how much the magnetic field sensitivity can be improved. That is, the gap cannot be made sufficiently large to markedly improve the magnetic field sensitivity of the SVM coil.
Japanese Laid-open Patent No. Heisei 11-162372 discloses an electron gun, in which slits are formed on side surfaces of electrodes of the electron gun that correspond to a position of an SVM coil. The slits are formed perpendicular to a direction electron beams travel. An electric field generated by the SVM coil passes through the slits such that the entrance of an external electric field into a gap and the generation of eddy currents on surfaces of the electrodes are prevented. However, the formation of the slits on side surfaces of the electrodes is more difficult than can be justified by the advantages obtained. Such difficulties result in increasing overall manufacturing costs.